Study of the Combined Contributions of Porous Coverings and Mixture Surface Energies in Enhancing Pool Boiling

Abstract

Factors as the boiling fluid surface tension and the characteristics of the solid surface where the heat transfer takes place could be modulated for increasing the boiling heat flux. In this work was observed the increase in the boiling convective heat-transfer coefficient (h) from the participation of: (a) the use of a binary mixture at its critical micelle concentration (16% w/w ethanol-water); (b) the addition of the surfactant sodium-lauryl-sulfate (SLS) to this aqueous mixture; and (c) the use of a porous covering fabricated from stainless steel bands with void volume 0.25, pore diameter 0.8 mm and covering thickness 8 mm. The sequence of results allowed the calculation of the relative participation of these factors in h (and the related values of excess temperature), for power supply from 100 to 1000 W on the same heater cartridge for all the experiments. For boiling water on the bare heater, hmax bare heater = 8.27 W/cm2 K; for boiling water on the porous covering, hmax covering = 19.36 W/cm2 K; the boiling of the water-ethanol (16%) mixture on the porous covering produced hmax covering+cmc = 31.72 W/cm2 K; and the binary mixture with 100 ppm of SLS, hmax covering+cmc+surfactant = 38.07 W/cm2 K. Considering this value of hmax covering+cmc+surfactant as the sum of the contributions, the relative participation of the mechanical forces breaking the escaping bubbles through the covering is 29.13%; the surface energies associated to the formation of micelle structures 32.47%; and the surface energies from the surfactant 16.67%. Thus, the search of enhancing heat transfer should consider the boiling mixture composition as well as the porous covering design. A comparison of the results obtained with the covering developed in this work with some coverings developed in a previous work reveals that the geometry of the covering material could be the base for heat transfer enhancement.